In conventional units of this kind, it has been known to carry out various of the vehicle functions using hydraulic motors or actuators, but the relatively high cost of such techniques has restricted their use to selected functions only. We have found however that the advantages gained by using hydrostatic drives for essentially all of the vehicle functions outweigh the extra cost involved, particularly in the respect that with proper application of conventional hydrostatic engineering techniques, we find that a more versatile and effective unit can be produced which is nevertheless substantially easier to operate than conventional units.